The pressure sensor of the prior art utilizing a strain gage is formed by providing an insulator of polyimide and the like on a metallic diaphragm and forming a strain gage by evaporation on top, or by adhering an already-formed strain gage on a metallic diaphragm by an adhesive and the like.
The pressure sensor utilizing such a strain gage has an advantage that the output linearity is well and the structure of the electric circuit is relatively simple. However, the characteristics of the metallic diaphragm, for example, the flexural strength, the deformation hysteresis by the change in pressurization, and the permanent deformation by heat expansion and stress caused deterioration of accuracy and durability of the sensor.
Especially, as shown in FIG. 10, in a pressure sensor formed by cutting out from a metal block a clamp member 101, a diaphragm portion 102 and a pressure opening 103 having a screw portion 104 on the inner wall thereof for fixing a piping, further having a strain gage 105 formed on top of the diaphragm 102, the clamp member 101 and the diaphragm 102 are formed solidly, so strain will occur in the diaphragm 102 when being clamped by too much torque, resulting in unnecessary change in output.
Further, in order to detect the accurate pressure, there was a need to utilize dielectric pressure, since there was only a very thin insulating body between the metallic diaphragm and the strain gage.
In such a case, there was a need to put an insulating body between the metallic diaphragm and the strain gage, but this insulating body causes a drift phenomenon wherein the output characters will change by cause of change in contact between the diaphragm by temperature, and the deformation by the repeated displacement of the diaphragm.
Further, according to the structure of the strain gage, the permanent deformation or the breakage of the diaphragm may happen when too much stress is loaded to the diaphragm. The breakage of the diaphragm may cause serious damage like the emission of pressure medium.
On the other hand, a capacitance-operated pressure sensor is known as the pressure sensor utilizing insulators such as ceramic and the like to the diaphragm. These kind of pressure sensors are advantageous in that very high durability and repeatability of the measurement could be gained because of the characteristics of the material being used.
However, these type of pressure sensors have problems such as the method of detecting the change in distance between the electrodes by the capacitance being inferior to the potential linearity of output, the electric circuit being very complicated, the floating capacity being unstable, the manufacturing of ceramic elements of the sensing unit needing high accuracy, and so on.
Still further, a pressure sensor gained by forming and baking a thick-film resistor in a bridge state on a diaphragm formed of a ceramic plate by a screen print is known, as is disclosed in Japanese Patent Publication No. S62-12458. This pressure sensor will not be influenced by the capacity, but when the impressed pressure rises above a certain level, it will cause permanent deformation to the diaphragm, or even worse, break the diaphragm.
The object of the present invention, based on the problems mentioned above, is to provide a strain gage-type pressure sensor utilizing insulating thin plate of ceramic materials such as alumina and zirconia, silicon or crystal and the like, wherein too much deformation or breakage of the diaphragm is prevented.
Further, the present invention is aimed at providing a pressure sensor with high accuracy and high repeatability.